In optical communications and optical measurement, when light that has exited a semiconductor laser serving as the light source is reflected from the surfaces of members provided along the transmission path and returns to the semiconductor laser, laser oscillation becomes unstable. Optical isolators containing Faraday rotators that non-reciprocally rotate the light's plane of polarization are used to block such reflected returning light.
Recently, in the semiconductor laser modules utilized in optical communications, increasing use has been made of tunable laser light sources having a broad oscillation wavelength range. It is desired that the optical isolators used for tunable laser light sources exhibit a high isolation performance over a broad wavelength range.
When using a tunable laser light source, the method of increasing wavelength stability by placing the optical isolator on a temperature regulator (Peltier element) has hitherto been employed. However, to conserve power, temperature regulators are no longer used; moreover, the module interior is now often set to a high temperature. Hence, it is also desired that the optical isolators used in tunable laser light sources be suitable for a broad temperature range.
Given this desire, 1.5-stage optical isolators and two-stage optical isolators are generally used in modules that employ a tunable laser light source and in applications requiring a high reliability. Referring to FIG. 5, a 1.5-stage optical isolator is an optical isolator 50 having a first polarizer 51, a first Faraday rotator 52, a second polarizer 53, a second Faraday rotator 54, a third polarizer 55 and a half-wave plate 56 arranged in this order on a light transmission path (indicated by the symbol X in the diagram), and magnets 57 and 58 arranged on the periphery thereof. FIG. 6A and FIG. 6B show the direction of light polarization at each of a number of places a′ to g′ on the light transmission path in FIG. 5. The arrows at a′ to g′ in FIG. 6A and FIG. 6B indicates the direction of polarization as seen from the light input side.
In FIG. 6A and FIG. 6B, incoming light that is polarized in the vertical direction, in the course of passing through the first polarizer 51, the first Faraday rotator 52, the second is polarizer 53, the second Faraday rotator 54 and the third polarizer 55, becomes light polarized in the horizontal direction. The plane of polarization is then rotated 90° by a half-wave plate 56, whereupon the polarization directions for the input light and the output light become the same.
A semiconductor laser chip, an optical isolator and a waveguide-type modulator are often arranged in this order in a semiconductor laser module. Because the semiconductor laser chip has output polarization direction dependency and the wavelength-type modulator has input polarization direction dependency, the polarization direction of light entering the optical isolator positioned therebetween and the polarization direction of light leaving the isolator need to be made to agree.
Accordingly, it is desired that such a 1.5-stage optical isolator have a high isolation performance and also a smaller size and a lower cost, and so improved technology has been awaited.
To address this, JP-A 2004-233385 describes a repulsion-type 1.5-stage optical isolator in which two magnets are arranged with the polarities reversed and which does not use a half-wave plate. However, in this method, the two magnets repel one another, making assembly difficult and also making high reliability in fixing the magnets essential.